Sensors for the detection of two-dimensional and three-dimensional images are used in numerous applications, such as commercial video, medical applications, astronomy and instrumentation. The particular type of EM wave detection employed varies depending upon the context and requirements of a particular application, such as wavelength and acquisition speed. For example, in astronomy, light images are extremely dim, but acquisition times may be on the order of seconds, minutes or even hours. In medical applications, there are significant constraints on the size and power dissipation of sensors to allow insertion in the human body or, in general the use in a sterile environment. In commercial video, generally the most stringent constraints are acquisition time, quality of the image and overall device cost.
For EM wave detection in the visible, ultraviolet (UV) and infrared (IR) wavelengths, EM waves are typically detected using photo elements such as silicon photodiodes or photogates. The fundamental measurement principle is based on integrating a current generated by the photoelement across a capacitor, usually the parasitic capacitor of the photoelement. At the end of the integration period, the voltage present across the capacitor is read out, for example, through a sequential charge transfer used in charge coupled devices (CCDs).
One of the limitations of such systems is the apparent correlation between integration time and sensitivity of detection. The area of the photodiode determines both the absolute value of the current being generated for a given irradiation intensity per unit area and its inner parasitic capacitance. Increasing the size of the detector increases the current generated for a given intensity, but doing so increases the inner capacitance (which is used as an integrator) of the device. This in turn increases the number of charges (and time) needed to reach a measurable voltage level.
FIG. 1A is a block diagram that depicts a conventional sensor chip 100 that includes a pixel array 102 and associated circuitry for transferring the voltage accumulated in every pixel to the outside of pixel array 102. In the present example, pixel array 102 includes a row decoder module 104, a column decoder and bias module 106, an amplification and sampling module 108, an A/D conversion module 110 and an input/output (I/O) module 112. Random access to every pixel in pixel array 102 is performed by selecting a given row through row decoder 104 and by transferring the charge in all the pixels in this row to a sampling device, such as amplification and sampling module 108 (one for each column), that can be sequentially or simultaneously accessed. The voltage associated with the pixel charge is then accurately amplified and timely converted to a digital code by amplification and sampling module 108 and A/D conversion module 110, respectively. The resulting code can then be transmitted to the exterior of sensor chip 100 through appropriate logic.
FIG. 1B is a block diagram that depicts a conventional pixel arrangement 150 based on a photodiode. When the reset line (RST) is de-asserted, i.e., changed from a logic LOW to a logic HIGH, a reset transistor 152 stops conducting and a photodiode 154 is placed in reception mode. A reception mode consists of a state in which the photodiode is in reverse bias and it is thus ready to generate a current when incident photons are absorbed by its substrate. Reset transistor 152 injects a spurious current due to the accumulation of charges in its channel during the on phase that need be evacuated from the channel. The spurious current from reset transistor 152 charges Cd and other parasitic capacitances at node VO resulting in a sampled error voltage. An additional source of error voltage is due to the kT/C effect caused by Cd. kT/C effects are described in more detail hereinafter. The error voltage is conventionally canceled through a process known as (un)correlated double-sampling. This process consists of sampling VO through a shutter transistor 156, via a SHUT signal, before the EM waveform intensity IWF is received and subtracted from the final sampled voltage resulting from the integration process. In some cases it may be advantageous to sample the final voltage resulting from the integration process first, reset the pixel and again sample the error voltage. The latter is known as uncorrelated douple-sampling.
Assuming a square pulse of current generated by photodiode 104, the final voltage accumulated in the parasitic capacitances connected to node VO are as follows:VOUT(ti)=IdTH/Cd  (1)
However, since for a given intensity IWF, Id is directly proportional to the area of photodiode 154 (Ad) and Cd is directly dependent on Ad, then,VOUT(t1)∝IWFTH,  (2)where the only parameters that can be modified for a given technology are the pulse duration and intensity. Hence, a long integration time must be used to obtain a sufficiently strong signal, making this arrangement generally unsuitable for fast and dim EM waveforms.
Based on the foregoing, there is a need for improved EM wave detection systems. There is a particular need for improved EM wave detection systems that are suitable for low intensity EM wave applications while maintaining reasonably short acquisition times. There is yet another need for improved EM wave detection systems that are reproducible and therefore suitable for mass production and use in large pixel matrix applications.